Words of science: crepuscular

Like many other fields, science has its own style of communication full of specific jargon and guided by unwritten rules. Most of the posts on this blog focus on breaching this barrier to the public’s understanding and appreciation of science. In this series, I’d like to take another approach by highlighting scientific words which have escaped the confines of jargon to reach a broader appeal because of their sound or their evocative power as metaphors. Today’s word is crepuscular. [Previous words of science were petrichor, alluvium, and nychthemeron]

Crepuscular is actually a very easy word to explain: it’s just an adjective for twilight. In fact, it comes from the Latin word for twilight, crepusculum. Crepuscular is used to describe the many plants and animals that are active at dusk and dawn, including fireflies (which are actually beetles, not flies), moths, domestic cats, and rabbits. Not all crepuscular creatures are active at both dawn and dusk; some, like morning glory, are mainly active in the morning (matutinal) and others in the evening (vespertine), like evening primrose. A crepuscular lifestyle probably helps many animals escape the attention of predators, who are often nocturnal or diurnal; an obvious exception is house cats, which are probably responding to higher levels of human activity. Crepuscular plants evolved to take advantage of the plentiful pollinators at dusk and dawn; vespertine flowers, in particular, are often white but intensely fragrant, like the night-blooming jasmine (called “the scent of night” in Arabic). Incidentally, I learned the word ‘vespertine’ (from the Latin vesper=”evening star, evening”) while researching this post and immediately fell in love with it! It feels like the perfect word for all things evening — probably because of its resonance with ‘whispers’.

Beyond the world of biology, crepuscular is also used to describe rays of sunlight that stream through gaps in clouds or other obstacles. Crepuscular rays, which stand out against the unlit air around them, are most easily seen around twilight, when the contrast between light and dark is most obvious.

Twilight is a very special time. The twelve hour day comes from the decision of ancient Egyptians to divide the day into ten sunlit hours and two extra twilight hours. There’s a certain quality of light at dusk and dawn, when the sun is still illuminating the atmosphere even though the surface is shrouded in darkness. Twilight is certainly a wonderful word, bringing to mind the quiet stillness of a fading day, but it doesn’t fully capture the secrets of those hours. Crepuscular, with its gentle, crackling sound and tinge of unfamiliarity has just the right qualities to evoke that mysterious pause between night and day. The different feelings of the two words is nicely captured in their etymology; crepuscular ultimately derives from the Latin word creper, meaning dark, dusky, doubtful and uncertain, while twilight comes from Germanic roots which mean something like ‘half light’.

“Crepuscular” is probably more common in scientific writing than elsewhere, but it’s hardly surprising that such an evocative word has refused those constraints, breaking free into metaphors and literature:

“The story is told from a more joyous point of view — from a point of view comparatively humorous — and a number of objects and incidents touched with the light of the profane world — the vulgar, many-coloured world of actuality, as distinguished from the crepuscular realm of the writer’s own reveries — are mingled with its course.”

“I’ve been through their checkout and noted its resemblance to Hades – the crepuscular gloom, the dungeon lighting, the mile-long shuffling queue, the glum, sickly faces, the trolleys piled high with flat-pack cardboard units.”

“In the crepuscular lobby, a broad circle of monitors laid on their backs on the floor blinked up at a laser show spiraling across a tentlike scrim stretched just below the building’s blacked-out skylight.”

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3 thoughts on “Words of science: crepuscular”

Now you’re in my world! Many animals have developed crepuscular patterns of behavior to avoid interaction with humans and to reduce predation pressure, as many of their natural predators forage most intensively during the night (nocturnal) or during the day (diurnal). Apart from the relevance to predation, crepuscular activity in hot regions is often the most effective way of avoiding thermal stress while capitalizing on available light. Great post!